In the prior art total internal reflection (TIR) holograms are constructed from expanded laser beams as shown schematically in FIG. 1. One of these, the object beam 1, is directed through a mask transparency 2 to a holographic recording layer 3 on a substrate 4 in optical contact with a large prism 5. The other, the reference beam 6, is directed through another face of the prism 5 so that it is totally reflected from the surface of the holographic layer 3. The optical interference of the two beams is recorded by the layer's photosensitive material. Once fixed, the hologram can be reconstructed by irradiating it with a laser beam directed in the opposite direction to the original reference beam 6.
TIR holograms have the capability of recording and reconstructing images over large geometrical areas. With modern photopolymeric recording materials, TIR holograms have shown significant potential for high resolution photolithography.
One of the problems encountered when applying TIR holography to photolithography, such as for the micro-electronics industry, is that the brightness of the reconstructed images needs to be very uniform (preferably better than .+-.2%) over their areas. For this to be achieved with the prior art it is therefore desirable that at hologram recording the expanded laser beams have good uniformity across their wavefronts. Unfortunately this is difficult to achieve because of the natural variation in irradiance of a laser beam: most beams have a Gaussian intensity profile, making their edges less bright than their centre.
A more uniform lightfield at the recording layer could, in principle, be achieved by expanding the beam well beyond what is needed and using the centre only. However, beam uniformity achieved this way is only at the expense of the light available. It has been estimated that to achieve .+-.2% uniformity, only about 2% of the laser energy could be used and this would increase exposure times beyond practicality.
Uniformity of printing exposure could also be achieved by recording a hologram with a well-behaved efficiency non-uniformity (eg. Gaussian) across its surface and then, during printing, to compensate for the non-uniformity by scanning the reconstruction beam over the hologram area and varying either the beam's intensity or scan-speed as it scans. However, because of the compromises being made in both mean hologram efficiency and mean laser power, this would necessarily lead to longer print times and consequently a lower throughput, which are undesirable for many industrial applications. Also, the assumption of a Gaussian intensity profile can often be an over-simplification.
In addition, both the above methods for improving uniformity require low-aberration collimating lenses (or mirrors) operating in the UV (typically 364 nm) for generating the object and reference beams. For large diameter (eg. 8", 12", 20") holograms intended for, for instance, manufacturing large-area flat panel displays, this becomes most unattractive.
Another problem encountered in the manufacture of large area TIR holograms for microlithographic applications relates to the depth of focus of the reconstructed images. In order that a TIR hologram correctly prints an image onto, for instance, a silicon wafer, it is necessary that the wafer's surface be accurately positioned with respect to the projected image. If the wafer surface is either a small distance in front of or behind the image, the printed image will be out-of-focus. Typically, if the features in the image are of dimension .about.0.5 .mu.m, the wafer surface needs to be positioned to .about..+-.0.2 .mu.m accuracy. In order to facilitate this, it is desirable that all features in the reconstructed image lie at the same distance from the hologram's surface. Using the prior art this demands that the separation of the object mask and holographic layer during recording be the same (to .about..+-.0.1 .mu.m) across their area. However, because of the difficulty in obtaining sufficiently flat object masks and sufficiently flat recording layers, and the difficulty in supporting them in such a way that they remain flat during hologram recording, the degree of parallelism required becomes impractical.
In seeking a solution to the above problems it is of prime importance to take into account the extreme sensitivity of hologram formation to mechanical (and other) instabilities. A hologram is a recording of an optical interference pattern and so will only be formed successfully if at each point on the recording surface the relative phases of the interfering object and reference beams are substantially constant during the exposure process. To quantify this, the relative phases should be constant to preferably better than 2.lambda./10, meaning that the relative lengths of the object and reference beam optical paths to any point on the holographic layer should not change by more than .about.30 nm during the recording operation; if this condition is violated the interference pattern will be "washed out" and the hologram will be lost.